Ultrasonic transducers are devices that convert energy into sound, typically in the nature of ultrasonic vibrations—sound waves that have a frequency above the normal range of human hearing. One of the most common types of ultrasonic transducers in modern use is the piezoelectric ultrasonic transducer which converts electric signals into mechanical vibrations. Piezoelectric materials are materials, traditionally crystalline structures and ceramics, which produce a voltage in response to the application of a mechanical stress. Since this effect also applies in the reverse, a voltage applied across a sample piezoelectric material will produce a mechanical stress within the sample. For example, activation of some piezoelectric materials results in a change of shape with up to a 4% volumetric variance. Suitably designed structures made from these materials can therefore be made that bend, expand, or contract when a current is applied thereto.
Many ultrasonic transducers are tuned structures that contain piezoelectric (“piezo”) ceramic rings. The piezo ceramic rings are typically made of a material, such as lead zirconium titanate ceramic (more commonly referred to as “PZT”), which have a proportional relationship between their applied voltage and mechanical strain (e.g., thickness) of the rings. The supplied electrical signal is typically provided at a frequency that matches the resonant frequency of the ultrasonic transducer. In reaction to this electrical signal, the piezo ceramic rings expand and contract to produce large-amplitude vibrational motion. For example, a 20 kHz ultrasonic transducer typically produces 20 microns of vibrational amplitude. The electrical signals are often provided as a sine wave by a power supply that regulates the signal so as to produce consistent amplitude mechanical vibrations and protect the mechanical structure against excessive strain or abrupt changes in amplitude or frequency.
Typically, the ultrasonic transducer is connected to an ultrasonic booster and a sonotrode (or “horn”), both of which are normally tuned to have a resonant frequency which matches that of the ultrasonic transducer. The ultrasonic booster, which is structured to permit mounting of the ultrasonic transducer assembly (or “stack”), is typically a tuned half-wave component that is configured to increase or decrease the vibrational amplitude passed between the converter (transducer) and sonotrode (horn). The amount of increase or decrease in amplitude is referred to as “gain.” The horn, which is oftentimes a tapering metal bar, is structured to augment the oscillation displacement amplitude provided by the ultrasonic transducer and thereby increase or decrease the ultrasonic vibration and distribute it across a desired work area.
Typically, all of the mechanical components used in an ultrasonic transducer assembly must be structured so that they operate at a single resonant frequency that is near or at a desired operating frequency. In addition, the ultrasonic transducer assembly must often operate with a vibrational motion that is parallel to the primary axis (i.e., the central longitudinal axis) of the assembly. The power supply for the stack generally operates as part of a closed-loop feedback system which monitors and regulates the applied voltage. Such a system works well when the ultrasonic assembly vibrates in response to a single mechanical resonance in a direction parallel to the primary axis of the ultrasonic transducer stack.
For some applications, it is desirable to ultrasonically vibrate non-tuned structures that have one or more non-axial mechanical resonant frequencies. The purpose of doing so may be to reduce friction when installing a non-tuned structure into or onto a larger assembly, or to reduce friction between a non-tuned structure and a material flowing through that structure, such as in a production environment. The presence of non-axial mechanical resonances in a typical non-tuned structure will often result in such resonances existing in or otherwise transferring back to the stack. Such resonances typically are random in nature rather than the expected single axial resonance on a fully tuned stack. As a result, the strain of the piezoelectric rings contained in the ultrasonic transducer tends to be more erratic. This tends to produce unrepeatable results when an ultrasonic power supply is used to regulate vibrational amplitude. There is therefore a need for an ultrasonic transducer assembly that is adapted to minimize or eliminate erratic strain on the piezoelectric rings which, in turn, will help to ensure more consistent, repeatable output by the stack during normal operation thereof.